In recent years, there have been increased demands placed on display devices used to display images such as images generated by television, computer displays and the like. These demands include demand for larger images, while at the same time presenting these larger images with increased brightness, contrast, and perhaps most importantly, resolution. The most common method of expressing the resolution of a display device is by expressing the pixel density of a display device. A "pixel" is the basic `picture element` of an image (sometimes referred to as `pels`). The term pixel usually applies to the quantification of electronic images, which are composed of an array of pixels that each define a tiny portion of the image. This array of image picture elements is usually specified by a vertical number and a horizontal number, the product of which is the total number of pixels.
The need for large, high resolution display devices is becoming even more important because the United States and other countries are in the process of shifting from an analog, low resolution television delivery system, to a digital, high resolution delivery system, sometimes referred to as "high-definition television", or "HDTV". In terms of resolution, the current television delivery system in North America, known as NTSC (this format was developed by the National Television Standards Committee-hence the format has been named NTSC), has a resolution of approximately 425 by 565 pixels, thereby providing approximately 240,125 total pixels. A typical cable television system delivers even less resolution, approximately 350 by 466 pixels (163,100 total pixels). While there are as many as eighteen different formats proposed for digital television, there are approximately three different resolutions likely to be used by broadcasters and cable companies. These formats are base digital television, 480 by 640 pixels (307,200 total pixels), low HDTV, 720.times.1280 pixels (921,600 total pixels), and high (or full) HDTV, 1080 by 1920 pixels (2,073,600 total pixels).
Thus, a television capable of displaying full HDTV resolution must have the ability to display nearly nine times as much picture information (i.e., nearly nine times as many pixels) as current NTSC broadcasts require. This dramatic increase in resolution places demands on both the "engine" used to create the image, and the screens used to display the image. Current television display technology is not capable of efficiently displaying full HDTV resolution. By far the most popular large screen television system is the rear projection television, known as RPTV. A typical RPTV uses three cathode ray tubes that project picture data onto the rear of a transmission screen. The screen then distributes the picture data into an image viewing field, within which the viewer can see it. It is anticipated that other display technologies will be developed that are capable of higher resolution than projection cathode ray tube technology. Examples of newer projector technologies include liquid crystal displays (LCDs), which are often implemented with transmissive thin film transistors (TFT), and reflective silicon projectors (often referred to as digital light valves). These new technologies also promise to be less expensive than cathode ray tube technology, and importantly, deliver the projected image beam to the screen using only one projection lens instead of the three lenses common to CRT projectors.
Thus, as newer high performance projector engines are developed, a significant limiting factor in displaying high-resolution television images is the screen upon which the projector transmits picture data. Virtually every RPTV sold today utilizes what is known as a tooled micro-optic, hybrid diffusion/refractor type screen like those disclosed in U.S. Pat. Nos. 4,536,056 and 4,490,010 and available from the DNP Company of Japan. This screen is useful only with 3-lens CRT projectors and has no direct application to the new single-lens projectors. While these prior art screens are capable displaying the relatively low-resolution NTSC picture, they are not capable of displaying full HDTV images without being made impracticably large.
Thus, there is a strong need for a new type of screen that is capable of displaying high definition television signals with high resolution while providing high contrast, flexible viewing angles and brightness containment characteristics suitable for viewing in a room with large amounts of ambient light. Note that the term "containment" is a term used to describe `boundary control`, or how much of the image light is contained in the intended viewing field.
In U.S. Pat. No. 4,241,980 to Mihalakis ("the Mihalakis '980 patent), a reflection or transmission screen was disclosed where substantial boundary control of the reflected or transmitted light could be achieved while at the same time substantially excluding extraneous light. Thus, the Mihalakis '980 patent disclosed a screen that considerably increased the quantum of incident light comprising the image that was transmitted to the far-field viewing zone relative to surrounding lighting conditions. This was achieved by the construction of a repeating optical array element combining the lens focusing power of individual concave and convex elements into one single element, a plurality of which comprised the projection screen. This optical array element, which was based on compound optical curvatures in both planes perpendicular to the optical axis through its combination of convex and concave focusing properties, provided a higher degree of light distribution field control over previous element designs.
Fundamental to the screen disclosed in the Mihalakis '980 patent as well as to other prior art was the mathematical construction of lens or focusing elements with curvature functions that are compound in the two axes perpendicular to the optical axis. This is equivalent to stating that the curvature function in the direction of one principal axis perpendicular to the optical axis is modulated in the direction of the other principal perpendicular axis. This results in a mixed, or compound curvature function along all other directions between the two principal axes of the surface and between any two parallel cross-sections of the surface.
While the projection screen disclosed in the Mihalakis '980 patent represented an advance in gain distribution properties over earlier projection screens, it had substantial limitations in its overall efficacy factors when used with high-resolution image and data projector components which became available after the development of the screen disclosed in the Mihalakis '980 patent. The compound optical surface curvature and combined convex-concave element functions fundamental to the Mihalakis '980 patent are insufficient in critical factors such as farfield curvature, projector pixel resolution transmission, attainable optical specular polish and image contrast enhancement. These deficiencies are caused by the inherent properties of the screen surface disclosed in the Mihalakis '980 patent.
When used as a rear-projection transmission screen, the limitations of the screen disclosed in the Mihalakis '980 patent are due in total to its mathematically compound surface curvature construction, which is very important to the art disclosed in the Mihalakis '980 patent. In particular, these limitations in Mihalakis '980 and other relevant prior art are:
1) Limitations in the efficacy of its gain characteristic, which is caused by field curvature in the gain distribution far field. Field curvature reduces the screen's applicability and efficiency.
2) Limitations in the attainable specularity of the compound modulations of the optical surface. Inadequate specularity reduces the applicability and efficacy of a screen because it results in low light output (perceived by the viewer is low brightness). PA1 3) Limitations in the element's ability to optically compress its focused image through an optional contrast mask on the exit surface. PA1 4) Limitations in the element's surface's ability to transmit high resolution images having high pixel densities. PA1 1) Low field curvature, thereby allowing more efficient containment of the image brightness within the desired viewing angles. PA1 2) High specularity, which will provide improved "gain" and contrast to the projected image. This is extremely important because most televisions are located and viewed in rooms with large amounts of ambient light. PA1 3) The ability to optically compress the focused image through a contrast masks on the exit surface, which is a technique used by the screen manufacturer. PA1 4) The ability to transmit high resolution images, i.e., high pixel densities.
Thus, the screen disclosed in the Mihalakis '980 patent, as well as other prior art screens, have significant limitations.
Thus, there is a need for screen for a rear projection television with the following characteristics:
The present invention discloses a screen for use with rear projection televisions, computer monitors and other displays that provides these characteristics.